U.S. Pat. No. 3,369,137 in the name of R. A. Sanger describes a motor-pump unit in which the motor portion is filled with a fluid to protect it from the surrounding medium. At the top end of the motor portion, the unit includes a dynamic sealing system separating the motor portion and its protective fluid from the pump portion which is driven by the motor portion and which is in direct contact with the surrounding medium. Underneath the motor portion and beyond the end of its shaft, there is a flexible compenstation tank or bag communicating with the motor portion. Said flexible tank is subjected both to the pressure of the surrounding medium and to the action of spring compression means arranged to ensure that fhe fluid pressure inside the flexible tank, and hence inside the motor portion, is greater than the pressure of the surrounding medium. Thus any leakage which may occur through the dynamic seal will be protective fluid leaking from the motor portion into the well, rather than surrounding medium leaking from the well into the motor portion.
In very deep wells, motor-pump units are generally constituted by an assembly of several very long motors of small diameter, eg. three motors each of which is eight to nine meters long, with several coupled pumps mounted above them. While such a unit is being lowered down the well, its temperature rises, and there comes a point where the pressure inside the motor portion is excessive because of the expansion of the oil in the flexible tank. Thus such flexible tanks are not suitable for use in some the deeper and hotter wells in which motor-pump units are to be installed.
Preferred embodiments of the present invention mitigate this drawback and make it possible to use such flexible tanks at greater depths and temperatures than before.